This invention relates to a heat-insulating layer in an internal combustion engine to prevent unwanted lowering of the temperature of combustion gas in combustion chambers or exhaust ports.
Regarding internal combustion engines, particularly automotive engines, the employment of a heat-insulating structure or a heat-insulating coating is effective not only for enhancement of thermal efficiency of the engine but also for decrease in the amount of unburned hydrocarbons (HC) emitted into the atmosphere as an undesirable component of the exhaust gas. Furthermore, when the exhaust system of the engine comprises either a thermal reactor or a catalytic converter to purify the exhaust gas thereby to meet current emission standards, it is desirable to minimize lowering of the exhaust gas temperature before the entrance of the exhaust gas into the reactor or the converter because such a device requires a certain minimum temperature to exhibit its oxidation or conversion ability and exhibits its full ability at considerably high temperatures.
For the purpose of maintaining high exhaust temperatures in internal combustion engines, it has been proposed and sometimes put into practice to cover the wall surfaces of combustion chambers, top face of each piston and/or wall surfaces of exhaust ports with a ceramic material low in heat conductivity, either by attachment of a ceramic plate directly to a surface to be covered or by a flame or plasma spraying technique. However, in practical applications this heat-insulating method involves a serious problem that a ceramic layer formed on a metal surface is liable to crack, break and even separate (at least fragmentarily) from the metal surface due to shocks and vibrations experienced during operation of the engine and a difference in thermal expansion characteristics between the ceramic and the metal. Of course this means an insufficient service life of the heat-insulating layer. As a matter of more seriousness, the service life of a combustion chamber is shortened significantly when fragments of the injured ceramic layer fall into the combustion chamber.
Accordingly there is an earnest desire for a method of producing a heat-insulating layer which can withstand severe environmental conditions in internal combustion engines, that is, a novel type of method for strong, reliable and durable bonding between a metal member and a ceramic material.